Chiller systems typically incorporate the standard components of a refrigeration loop to provide chilled water for cooling a designated building space. A typical refrigeration loop includes a compressor to compress refrigerant gas, a condenser to condense the compressed refrigerant to a liquid, and an evaporator that utilizes the liquid refrigerant to cool water. The chilled water can then be piped to the space to be cooled.
Chiller systems that utilize so called centrifugal compressors can typically range in size, for example, from ˜100 to ˜10,000 tons of refrigeration, and can provide certain advantages and efficiencies when used in large installations such as commercial buildings. The reliability of centrifugal chillers can be high, and the maintenance requirements can be low, as centrifugal compression typically involves the purely rotational motion of only a few mechanical parts.
A centrifugal compressor typically has an impeller that can be thought of as a fan with many fan blades. The impeller typically is surrounded by a duct. The refrigerant flow to the impeller can be controlled by variable inlet guide vanes (“IGV”s) located in the duct at the inlet to the impeller. The inlet guide vanes can operate at an angle to the direction of flow and cause the refrigerant flow to swirl just before entering the compressor impeller. The angle of the inlet guide vanes can be variable with respect to the direction of refrigerant flow. As the angle of the inlet guide vanes is varied and the inlet guide vanes open and close, the refrigerant flow to the compressor can be increased or decreased. In many applications, the inlet guide vanes can be variable ninety degrees between a fully closed position perpendicular to the direction of the refrigerant flow to a fully open inlet vane guide position in which the inlet guide vanes are aligned with the refrigerant flow. When the cooling load is high, the inlet guide vanes can be opened to increase the amount of refrigerant drawn through the evaporator, thereby increasing the operational cooling capacity of the chiller.
In order to meet all conditions of demand in the air conditioned space, the chiller system can vary the output capacity. At times of high cooling demand, the centrifugal compressor can run at maximum load or full capacity. At other times the need for air conditioning is reduced and the centrifugal compressor can be run at a reduced capacity. The output of the chiller system then can be substantially less than the output at full capacity. It is also desired to operate the centrifugal compressor at the most efficient mode for the capacity that is required at any given time in order to reduce the electrical consumption of the chiller system to the lowest possible amount for the given load. The most efficient point of operation for a centrifugal compressor has been found to be near a condition known as surge. Operation in the surge condition, however, can be undesirable as this can cause damage to the centrifugal compressor.